The present invention relates to a playback head of magnetoresistive type suitable for reproducing signals recorded in a perpendicular magnetic recording medium, or more in particular to a perpendicular magnetic recording and reproducing system using the playback head and a magnetic recording and reproducing device using the perpendicular magnetic recording system.
It is known that the perpendicular or vertical magnetic recording system is expected to have a remarkably improved recording density as compared with the prior art longitudinal or horizontal recording system.
Examples of conventional perpendicular magnetic recording systems will be described below. FIG. 1 (a) shows a perpendicular head. Reference numeral 1 designates a main pole made of a ferromagnetic film such as permalloy. Explanation will be here by reference to a recording medium of the double-layered film type which is said to have a large output. Numeral 2 designates a perpendicular recording medium such as CoCr film with axis of easy magnetization in the direction perpendicular to the film surface, numeral 3 a lining layer of soft magnetic material such as permalloy, and numeral 4 a base film such as polyethylene terephathalate (PET).
Numeral 5 designates an auxiliary magnetic pole made of a ferromagnetic material such as ferrite, and numeral 6 a coil wound around the auxiliary magnetic pole 5.
When a signal current is applied to the coil 6 at the time of recording, the auxiliary magnetic pole 5 of the vertical magnetic head of this construction is magnetized, and the resulting magnetic fluxes reach the main pole 1 through the soft magnetic layer thereby to strongly magnetize the forward end of the main pole 1. As a result, the recording film 2 of CoCr is magnetized in the perpendicular direction.
The arrows in FIG. 1 (a) indicate the flow of magnetic fluxes. At the time of reproduction, a reproduction voltage is generated across the coil 6 through processes opposite to those mentioned above. This system has the advantage that since a very sharp perpendicular magnetic field is capable of being generated at the time of recording, an ideal perpendicular recording is possible. In spite of this advantage, the disadvantage of this system is a low reproduction efficiency since the magnetic circuit is open at the time of reproduction.
FIG. 1 (b) shows another example of the conventional systems, in which a ring head usually used for longitudinal recording is used for perpendicular recording. Numeral 7 designates a ring head core. In the recording process, the system utilizes vertical components of the magnetic fields generated by the ring head. The reproduction process, on the other hand, is performed in a manner similar to the conventional longitudinal recording. In the system, a considerable amount of oblique components of the magnetic field strength are generated by the ring head in the recording process, and therefore an ideal perpendicular recording is impossible, although considerably satisfactory recording characteristics are obtained if the anisotropy of the perpendicular recording medium is high. On the other hand, the reproduction efficiency is high due to the closed magnetic circuit construction.
The common disadvantage of these two conventional systems is the thickness loss of the main pole for the perpendicular head and the gap loss for the ring head. These losses are attributable to the thickness of the main pole and to the gap length of the ring head respectively as expressed below, and zero points arise in wavelength response (wavelength versus loss characteristic). This is illustrated in FIG. 2. ##EQU1## where .lambda..sub.on is the wavelength at zero point, l the gap length or the thickness of the main pole, n an integral number and .lambda. the wavelength. It will be seen from this that if the loss in a short wavelength region is to be reduced, it is necessary to reduce the gap length or, as the case may be, the thickness of the main pole. In either case, the reproduction efficiency is decreased proportionately.
If the number of turns of the winding is increased to improve the reproduction sensitivity, the self-resonance frequency is reduced by an increase of head inductance. With a decrease of recording wavelength, on the other hand, the signal frequency is increased, and therefore the reduction in the self-resonance frequency of the magnetic head is very disadvantageous for signal reproduction.
By reason of this limitation in reducing the gap length or the thickness of the main pole, the reproduction spectrum of such a head takes the form as shown in FIG. 2. In the case where a null point exists in the reproduction spectrum, the use of a region of shorter wavelength requires a special encoding system.
As a solution of this problem, an encoding system (partial response system) which permits the use of the reproduction head in the short wavelength region below or not longer than the zero-point wavelengths is under study. This encoding system has shortcomings that (1) a special encoding process is required, (2) the use of a high-order peak of wavelength response results in an interruption of the wavelength response spectrum, and (3) the output of the high-order peak is much lower than in the long wavelength region.
Another great common problem of electromagnetic induction type heads is that in the case of low relative speeds of the head and the recording medium, the reproduction output voltage is low, thereby necessitating an increase in the number of turns of winding. In the multi-track construction with a multiplicity ofmagnetic heads in juxtaposition, on the other hand, the space occupied by the winding poses the problem. Further, in the construction using thin film techniques, the number of turns in a winding is limited, thus making it impossible to realize a high-sensitivity reproduction head.
As a solution to these problems, a head utilizing magnets-resistive effect (hereinbelow abbreviated as MR head, and magneto-resistive element as MR element) is of interest. The conventional MR heads include an MR-element alone type MR head in which current is supplied longitudinally of an MR element in strip form, the MR element is arranged perpendicular to a recording medium, and a signal magnetic field enters the surface of the element in the direction perpendicular to the longitudinal direction. In the MR head of this type, it is known that the wavelength response characteristic, attributable solely to the head construction, is determined by the width w of the magneto-resistive element. In order to sufficiently reduce the wavelength loss, the element width w is required to be almost equal to the wavelength .lambda.. This is very disadvantageous in respect of the working tolerance for manufacture of a head intended for short wavelengths as well as for wear tolerance of the magnetic head. Another conventional type of MR head is of a shield type, in which magnetic materials of high magnetic permeability are disposed on both sides along the thickness of an magneto-resistive element. The MR head of this type exhibits substantially the same wavelength response as the conventional winding head of ring type and is known to be usable with high sensitivity up to a considerably short wavelength. Nevertheless, the magnetic and electric isolation is required between the magneto-resistive element and the high-permeability magnetic materials on the sides thereof, and the thicknesses g.sub.1 and g.sub.2 of the insulation layers therebetween correspond to the gap length of the conventional ringtype winding head. Further, the gap loss of g.sub.1 is multiplied by the gap loss of g.sub.2 approximately, and therefore, if the gap loss for the short wavelength is to be sufficiently reduced, both the values g.sub.1 and g.sub.2 are required to be reduced extremely. Under this condition, it is very difficult to form a narrow gap free of magnetic or electric leaks.
As will be seen from the above description, in spite of the fact that the perpendicular magnetic recording system has a superior recording characteristic, a superior playback head for the system which is capable of reproducing very short wavelengths efficiently has been not yet developed. The available perpendicular magnetization reproduction system or the perpendicular magnetic recording and reproducing apparatuses equipped with such a playback head are not satisfactory.